1. Field of the Invention
The present invention relates to techniques, both method and apparatus, for monitoring intermediate regenerative repeaters connected between two line terminal devices in a digital transmission system, in which a telemetry signal is transmitted over the same signal path as the digital signal, however, in a different frequency range.
2. Description of the Prior Art
In digital transmission systems it is necessary to employ intermediate regenerative repeaters. In order to locate defective intermediate regenerative repeaters, there are basically two different classes of locating systems. In one class, the communication transmission must be interrupted for the purpose of error location . In this class are all methods having a loop connection in the individual intermediate regenerative repeaters. These methods are very economical and permit an accurate error location. However, this technique can require a longer interruption of operation for errors which occur infrequently or briefly.
In the second class of locating systems, the error location occurs during operation. This has the advantage that no interruption is necessary in order to search for an error and that, in addition, one can recognize a slow impairment of a regenerative repeater at a longer period of time prior to an operational breakdown. These locating systems require a redundant transmission signal which, on the basis of an error-recognizing code, permits a local error evaluation, and a telemetry system for the transmission of the result to an evaluating station. Likewise, naturally, also the complete drop-out of the received digital signal or the failure of the current supply can be transmitted. Although these locating systems have the disadvantage of a higher apparatus expense in the individual intermediate regenerative repeaters, they offer so many advantages in terms of operation, in exchange, that they are increasingly employed, particularly in the case of broadband systems. Such methods are already described in publications.
Therefore, in the journal "Philips Telecommunication Review", Vol. 37, No. 3, August 1979, pp. 144-169, a 140 Mbit/s "coaxial transmission system" is described which is equipped with a monitoring installation. From a main station, the error information of the individual intermediate regenerative repeaters can be polled by way of a keyboard.
In the case of this method, an addressing of the individual intermediate regenerative repeaters is necessary. This requires expensive procedures and leads to unnecessary time delays in the case of error communication. Moreover, different addresses must be set in the case of the intermediate regenerative repeaters.